IT HAS long been recognized that the kidney is intimately involved in the regulation of systemic arterial pressure. The capacity of the kidney to excrete sodium is possibly the most important determinant of arterial pressure (1). In 1898, Tigerstedt and Bergman identified a renal pressor substance subsequently named renin (2), and with the recent availability of assays for the measurement of renin, this substance has been widely studied in hypertensive diseases. Additionally, the kidney may produce a number of other vasopressor and vasodepressor substances. These substances have not been studied as extensively as renin, and their role in the pathogenesis of hypertension has not been clearly defined. The purpose of this paper is to describe the renin-angiotensinaldosterone axis and to review evidence for its contribution to the regulation of both normal arterial pressure and elevated arterial pressure in selected hypertensive diseases. Renin is produced within the portion of the renal afferent arteriole that is adjacent to an early segment of the distal convoluted tubule, the macula densa (3, 4).